1. Field of the Invention
The present invention relates to detection of skew of a sheet in a sheet conveying device equipped in an image forming apparatus, such as a copying machine or a printer, or in an image reading apparatus, such as a scanner.
2. Description of the Related Art
In general, in an image forming apparatus, such as a copying machine or a printer, during printing, a sheet is conveyed to an image forming section, and printing is performed on the conveyed sheet. In this sheet conveyance, a sheet is sometimes conveyed obliquely with respect to a conveying direction, i.e. so-called skew sometimes occurs. If skew of a sheet occurs, when the image forming section transfers a toner image onto the sheet, the position of the image is displaced with respect to the sheet. As a result, occurrence of skew of a sheet sometimes prevents the printing operation from being performed with high accuracy. To overcome this problem, conventional sheet conveying devices are each equipped with a skew correction mechanism for correcting skew of a sheet.
On the other hand, to perform printing at high speed, it is necessary to promptly perform correction of skew of a sheet. To this end, there has been proposed a skew correction mechanism using a so-called active registration method, as a skew correction mechanism. In this active registration method, skew caused when a sheet is fed is corrected while conveying the sheet (see e.g. Japanese Patent Laid-Open Publication No. H04-277151). This mechanism increases the speed of printing by promptly performing correction of skew of a sheet.
By the way, in an image forming apparatus, image formation on various types of sheets is desired. For example, it is desired that image formation (printing) can be also performed on a sheet which is not always a rectangle, such as an index tab sheet (also referred to as the tab sheet). Note that the index tab sheet is intended to mean a sheet on which an index tab is formed on a sheet edge, for entry of headings or the like for the purpose of classification. Further, there has been proposed a skew correction method for sheets not having a rectangular shape, such as index tab sheets.
FIG. 10 illustrates an example of a conventional skew correction mechanism. In the illustrated skew correction mechanism, two skew-detection sensors 82 and 83 are disposed along a direction orthogonal to a direction of conveying a sheet 81. The skew correction mechanism further includes a pair of conveying rollers 84 and 85 the respective conveying speeds of which are variable.
In the illustrated example, the sheet 81 is an index tab sheet, and sheet shape information indicative of a dimension X (dimension in the conveying direction) of an index tab 81a is registered in a memory or the like in advance. Further, a position of the index tab 81a on the index tab sheet is registered in the memory or the like in advance as position information.
In this mechanism, the skew detection sensors 82 and 83 each detect an edge of the index tab sheet 81 to obtain an amount of skew of the index tab sheet 81 according to the detection result X′, the above-mentioned dimension X (sheet shape information), and the position information. Then, the conveying speeds of the conveying rollers 84 and 85 are controlled, respectively, according to the amount of skew, to thereby correct the skew of the index tab sheet.
On the other hand, there has been proposed a technique in which to detect skew of a sheet, line sensors are provided in a sheet width direction and a shape of an edge of the sheet is detected by the line sensors (see e.g. Japanese Patent Laid-Open Publication No. 2003-146485). In Japanese Patent Laid-Open Publication No. 2003-146485, the shape of the edge of the sheet detected by the line sensors is subjected to image processing to thereby calculate an amount of skew of the sheet, whereby correction of skew of the sheet is performed.
Incidentally, when a plurality of index tab sheets are compared with each other, index tabs are not formed on the same position on the respective index tab sheets. More specifically, the index tabs are formed in a manner displaced on an index tab sheet-by-index tab sheet basis such that headings or the like written in the respective index tabs are easily confirmed when the plurality of index tab sheets are arranged one upon another.
When correcting skew of each index tab sheet formed as above, it is necessary to know whether or not an index tab passes a skew detection sensor in advance. For this reason, the user is required to designate whether or not a sheet is an index tab sheet, and further set the position of each index tab, the dimension of the same, and so forth, in a detailed manner, in the image forming apparatus.
Therefore, there is a problem that the user is required to perform troublesome operations when he/she intends to perform the skew correction for index tab sheets in order to perform printing with high accuracy at high speed.
The above-mentioned problem is also caused when original documents, which are index tab sheets, are consecutively read. More specifically, when the original documents are set on a document tray of an image reading apparatus so as to be read by the apparatus, the documents are conveyed from the document tray to a document reading position, but if it is intended to perform the skew correction at this time, the user is required to perform the troublesome operations described above.